Modeling of Ti-W Solidification Microstructures Under Additive Manufacturing Conditions

Additive manufacturing (AM) processes have many benefits for the fabrication of alloy parts, including the potential for greater microstructural control and targeted properties than traditional metallurgy processes. To accelerate utilization of this process to produce such parts, an effective computational modeling approach to identify the relationships between material and process parameters, microstructure, and part properties is essential. Development of such a model requires accounting for the many factors in play during this process, including laser absorption, material addition and melting, fluid flow, various modes of heat transport, and solidification. In this paper, we start with a more modest goal, to create a multiscale model for a specific AM process, Laser Engineered Net Shaping (LENS™), which couples a continuum-level description of a simplified beam melting problem (coupling heat absorption, heat transport, and fluid flow) with a Lattice Boltzmann-cellular automata (LB-CA) microscale model of combined fluid flow, solute transport, and solidification. We apply this model to a binary Ti-5.5 wt pct W alloy and compare calculated quantities, such as dendrite arm spacing, with experimental results reported in a companion paper

Finishing the euchromatic sequence of the human genome

The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead

Multiscale Design of Materials

Current methods of materials development, relying mostly on experimental tests, are slow and expensive, often taking over a decade and costing many millions of dollars to develop and certify new materials for critical applications. Finding new approaches for materials development is essential. Moreover, it will be increasingly important for materials development to be integrated into overall product design and development. We discuss a program in which we are creating a framework to link multiscale materials information to do design across scales, based on an existing computational design platform, VE-Suite. We present the basic framework of our program and discuss progress to date. Since our goal is multiscale design, part of our efforts are focussed on creating new visualization tools and facilities to enhance the design process. We review progress in this area as well.</p

Dislocation Dynamics Simulations of the Bauschinger Effect in Metallic Thin Films

Three-dimensional dislocation dynamics simulations were used to examine the role of surface passivation on the plasticity of thin films. A simple line-tension model was used to model the dislocation transmission cross grain boundaries. We find that passivated thin films have a higher hardening rate and strength than freestanding films and that the hardening rate increases with decreasing film thickness. Under unloading, passivated films exhibit a significant Bauschinger effect in which reverse plastic flow occurs during unloading. The Bauschinger effect is enhanced by an increasing pre-strain or by decreasing the aspect ratio of the film. The reverse motion of dislocation pile-ups and the collapse of misfit dislocations were found to be responsible for the observed Bauschinger effect in passivated films. The predicted deformation behavior is in excellent agreement with that seen experimentally

Dislocation Dynamics Simulations of Plasticity in Polycrystalline Thin Films

3-D discrete dislocation dynamics simulations were used to investigate the size-dependent plasticity in polycrystalline, free-standing, thin films. A simple line-tension model was used to model the dislocation transmission cross grain boundaries. At a constant film thickness, the total dislocation density and the strength increase as grain size decreases. The yield stress scales with grain diameter with a power law, with an exponent that varies with both film thickness and grain size for thicker films. In addition, the yield strength of films scales proportionally to the reciprocal of thickness and matches experiment results well. A spiral source model was developed that relates the strength of films to the statistical variation of the spiral source length, and accurately predicts the size-dependent strength in polycrystalline thin films

Structural Transformation in Monolayer Materials: A 2D to 1D Transformation

Reducing the dimensions of materials to atomic scales results in a large portion of atoms being at or near the surface, with lower bond order and thus higher energy. At such scales, reduction of the surface energy and surface stresses can be the driving force for the formation of new low-dimensional nanostructures, and may be exhibited through surface relaxation and/or surface reconstruction, which can be utilized for tailoring the properties and phase transformation of nanomaterials without applying any external load. Here we used atomistic simulations and revealed an intrinsic structural transformation in monolayer materials that lowers their dimension from 2D nanosheets to 1D nanostructures to reduce their surface and elastic energies. Experimental evidence of such transformation has also been revealed for one of the predicted nanostructures. Such transformation plays an important role in bi-/multi-layer 2D materials

Plastic Deformation Mechanisms of FCC Single Crystals at Small Scales

Three-dimensional (3-D) dislocation dynamics simulations were employed to examine the fundamental mechanisms of plasticity in small-scale face-centered cubic single crystals. Guided by the simulation results, we examined two distinct modes of behavior that reflect the dominant physical mechanisms of plastic deformation at small scales. We found that the residence lifetimes of internal dislocation sources formed by cross-slip decrease as the system size decreases. Below a critical sample size (which depends on the initial density of dislocations) the dislocation loss rate exceeds the multiplication rate, leading to the loss of internal dislocation sources. In this case nucleation of surface dislocations is required to provide dislocations for deformation and the starvation hardening mechanism becomes the dominant deformation process. When the sample is larger than a critical size multiplication of internal dislocation sources provides the dominant mechanism for plastic flow. As the strain is increased the rising dislocation density leads to reactions that shut off these sources, creating exhaustion hardening

Simulations of the Effect of Surface Coatings on Plasticity at Small Scales

Three-dimensional dislocation dynamics simulations were employed to examine how hard coatings affect plastic deformation in micron- and submicron-sized, single-crystal pillars ( micropillars ) of nickel. Cross-slip of dislocations in the coated samples was found to be necessary for the formation of banded structures and subcells. Our simulations thus offer an explanation for both the significant increases in compressive strength and the higher strain-hardening rate as well as formation of banded structures in coated micropillars

Discrete Dislocation Dynamics Simulations of Plasticity at Small Scales

Discrete dislocation dynamics simulations in three dimensions have been used to examine the role of dislocation multiplication and mobility on the plasticity in small samples under uniaxial compression. To account for the effects of the free surfaces a boundary-element method, with a superposition technique, was employed. Cross-slip motion of the dislocation was also included, and found to be critical to the modeling of the dislocation behavior. To compare directly with recent experiments on micropillars, simulation samples at small volumes were created by cutting them from bulk three-dimensional simulations, leading to a range of initial dislocation structures. Application was made to single-crystal nickel samples. Comparison of the simulation results and the experiments are excellent, finding essentially identical behavior. Examination of details of the dislocation mechanism illuminates many features unique to small samples and points directly to the importance of both the surface forces and cross-slip in understanding small-scale plasticity